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I have a question, what's difference between driver that support kernel version x.y and driver that patched to support kernel version x.y?

A patch only changes the glue code to match the kernel driver API.
If AMD releases a driver which supports a new kernel release they can, in principle,
make further changes to the driver code to optimise it for the new kernel.

You live in a fully unrealistic world. The fglrx kernel module is definitely not "optimized" for a new kernel, only build fixes have been applied, thats common practice. In this case you can look at the fix from AMD:

What you see that it is 1:1 the same, just an extra tiny modification thats not needed for Debian because the module build is done differently without that script. That code is standard, nothing that you have to think of long time.

Correction: Serious Sam 3 is not a Valve game, nor does it use the Source engine. The game is developed by Croteam and uses their in-house engine.
Team Fortress 2 is from Valve and does use the source engine.

2D KDE Performance

For anyone that has 2D problems with fglrx-catalyst at KDE 4.X environment try using Raster as 2D rendering mode... For me it showed great difference than native Xrender!
System Settings -> Desktop effects -> Advanced Tab -> Qt Graphic System that's for KDE 4.10 and 4.9 I think, for older versions it was a separate app in the system settings you had to install manually, Qt 2D something it was called!
Also at KDE 4.10 openGL ES 2 interfaces work so use them for Kwin...

Now to Serious Sam 3, there is absolutely no working fix, the lastest public beta from SS3 shows the menu correct, but the first logo is with artefacts. When you don't use gfxRestart() while the menu is shown (or via pressing Alt+Enter twice) then serveral textures like trees flicker in the level you start then.